I. Field of the Invention
The invention relates to the preparation of graft polymers and specifically to the preparation of graft polymers which are resistant to impact and aging.
II. Description of the Prior Art
Thermoplastic vinyl polymers or copolymers reinforced by the presence of an elastomeric constituent imparting to the end product a resistance to impact very much higher than that of the unreinforced polymer or copolymer, can be prepared in various ways. One of the oldest known methods consists of mixing the elastomeric component and the thermoplastic component mechanically. This process has been gradually replaced by methods designed to improve the reinforcement provided by the elastomer to the thermoplastic matrix, and there have appeared on the market graft polymers prepared in very different ways: in solution, in emulsion, in mass, or in suspension.
The first two methods, polymerization in solution and in emulsion, lead to products which are difficult to purify and whose end properties are affected by the presence of various additives added during the polymerization. Mass polymerization avoids any contaminations. However, it is difficult to maintain a homogeneous temperature in the medium since the medium becomes very viscous during mass polymerization, and this difficulty tends to make the reaction unfeasible on an industrial scale.
The method of polymerization in suspension is similar to the mass polymerization method in that each droplet of the organic medium to be polymerized, isolated in the suspension fluid, generally water, is the site of a polymerization. However, if the conditions of thermal exchange are distinctly more favorable to the good progress of the polymerization, it is impossible in this method to act directly on the material contained in each droplet by, for example, agitating it in a controlled manner.
Techniques have been sought which make it possible to associate prepolymerization in the mass and postpolymerization in suspension to take advantage of each of the methods at the time when it is found to be most useful. Thus, there has appeared the "mass/suspension" method, which is now well-known. See, for example, French Pat. No. 1,220,440.
U.S. Pat. No. 3,278,642 discloses a mass/suspension method for preparing terpolymers of the ABS type (acrylonitrile/butadiene/styrene) by grafting a matrix of styrene/acrylonitrile resin to an elastomer. These products yellow when they are exposed to light, and subsequently various improvements have been proposed to eliminate this deficiency. U.S. Pat. No. 3,448,175 discloses a similar method in which at least one supplementary addition of styrene is carried out during the postpolymerization in suspension, thus making it possible to attenuate the tendency of the products to yellow.
The mass/suspension method can only be carried out to the extent to which the elastomer employed -- polybutadiene or styrene/butadiene copolymer, for example -- is soluble in the initial mixture of the monomers. In the case of ABS type polymers, the initial mixture is composed of styrene and acrylonitrile in a proportion by weight close to the azeotropic composition of 75/25.
Despite the improvements made in this method of manufacture, the products obtained with polybutadiene as the elastomer serving as the basis for the grafting, are not sufficiently resistant to atmospheric aging, although they have an interesting surface appearance and have good resistance to impact. This poor resistance to aging, which is evidenced by rapid decline in properties with exposure over a period of time, is apparently attributable mostly to the oxidation of the elastomer. The oxidation of polybutadiene, like the oxidation of all diene elastomers, brings about the breaking of the elastomer chains and results in the weakening of the good mechanical properties of the initial thermoplastic material over a period of time. Improvements of the resistance to aging of a polymer of this type have been made by replacing the sensitive elastomer with another elastomer which is much more resistant to oxidation.
In general, an elastomer with a low unsaturation level has been used, and the presence of double bonds in the principal chain of the elastomer has been avoided as much as possible. Accordingly, elastomers which are completely or almost completely saturated have been recommended or used. Examples of such elastomers are the following: ethylene-vinyl acetate copolymers (EVA), acrylic elastomers, unsubstituted or halogenated butyl rubbers, ethylene/propylene copolymers (EPR) or ethylene/propylene copolymers containing a third monomer providing lateral double bonds (EPDM), chlorinated polyethylenes (CPE), polymers or copolymers of epichlorohydrin and .alpha.-olefin oxide, and silicone elastomers.
Nevertheless, although the use of such elastomers as a basis for grafting has been known for many years, such knowledge has not up to now given rise to true industrial achievements because of various difficulties related to the use of these products during the grafting reaction. In order to pinpoint these difficulties, which the present invention has made it possible to overcome, it is necessary to examine a graft mass polymerization, as known and widely practiced. In order to obtain a grafted polymer G from a thermoplastic resin R by the graft polymerization of a mixture of monomer M on to an elastomer E -- for example, an ABS type polymer in which R is a styrene/acrylonitrile resin, M is a mixture of styrene and acrylonitrile in given proportions, which proportions are usually close to the azeotropic composition of 75 parts by weight of styrene to 25 parts by weight of acrylonitrile, and E is an uncross-linked polybutadiene -- the elastomer E is dissolved in a mixture of monomer M, this solution being subjected to a free radical mass polymerization by any process, which may be catalytic or thermal, or may be initiated by radiation. This polymerization is continued up to a conversion rate which is sufficient to pass the phase inversion stage.
The mass polymerization may be continued up to the desired conversion rate, which is generally limited by the viscosity of the reaction medium. One may then either eliminate the unreacted monomers by devolatilization, or continue the polymerization by a process which makes it possible to accommodate the increasing viscosities of the reaction medium. For example, the mass obtained may be suspended in water and the polymerization may be continued until the desired degree of conversion, generally between 99 and 100%, is obtained.
This method can be used without difficulty so long as the elastomer E used is soluble in the mixture of monomers M at the rate at which one wishes to use it, but the method fails if this condition is not achieved. The elastomer is considered soluble at a given temperature in the mixture of monomers M if after agitation for several hours at a given temperature in a finely-divided state with the mixture of monomers M, a homogenous liquid whose content of dissolved solid matter corresponds to the percentage of elastomer introduced into the mixture M, is obtained.
It has not been possible up to today to apply this known technique to the production of materials requiring the grafting of certain mixtures of monomers to certain types of elastomers. It has not been possible, for example, to achieve by this method the grafting of a styrene/acrylonitrile resin to an ethylene and propylene elastomer such the EPR elastomers (ethylene/propylene rubber) or the EPDM elastomers (ethylene/propylene/diene monomer) because these elastomers, even though they are soluble in styrene, are not soluble in the azeotropic styrene/acrylonitrile mixture. This azeotropic composition is generally used since it imparts to the final graft polymer certain desirable properties. Numerous attempts have been made to overcome this obstacle. It has been proposed in, for example, U.S. Pat. No. 3,461,188, to add to the medium a third inert solvent which makes it possible to dissolve effectively the elastomer in the mixture of monomers and third solvent. However, this method requires the elimination of the third solvent during or at the end of the polymerization, a step which is complicated and expensive. It has also been proposed, according to U.S. Pat. No. 3,538,192, that one should produce a pseudo-solution of the elastomer in the monomers by using a dispersing agent which may be a grafted product of the same components obtained previously by a different method. This technique has the disadvantages of not being very easy to practice and of requiring the prior preparation of the dispersing agent.
U.S. Pat. No. 3,515,774 discloses a method of dissolving the elastomer in the vinyl aromatic monomer alone in prepolymerization until after the phase inversion and then introducing the acrylonitrile prior to passing into suspension. However, this method has a very serious drawback: at the level of acrylonitrile usually employed, which is about 25 parts by weight of acrylonitrile to 75 parts by weight of styrene, the copolymer formed after the introduction of the acrylonitrile is incompatible with the styrene polymer previously formed during the first phase of the graft polymerization. The result of this is that the product obtained has a certain number of defects, two of which are the poor appearance of the surface of parts manufactured from this product and the mediocrity of its mechanical properties, particularly at low temperature. A similar method, as described in French Pat. No. 2,211,482, in which an acrylonitrile fraction is introduced after the phase inversion, leads to products having a resistance to impact at low temperature which is very much lower than the resistance of the products obtained according to the process of this invention.